1. Field of the Invention
The present invention relates to a polarization beam splitter and a polarization conversion element.
2. Description of the Related Art
A polarization beam splitter typically includes a polarization split film sandwiched between a pair of transparent substrates. The polarization beam splitter is an optical element for splitting incident light into transmitted light and reflected light having polarization planes that are orthogonal to each other. The polarization beam splitter is used in various types of optical devices.
When the polarization beam splitter is used in an optical device using monochromatic light such as an optical pickup, favorable polarization-splitting properties can be realized relatively easily. However, when the polarization beam splitter is used as a polarization conversion element in a liquid crystal projector, it is required to have favorable polarization-splitting properties across the visible wavelength region.
Patent Document 1 discloses a polarization beam splitter in which a polarization split film is formed as a dielectric multilayer film and that realizes favorable polarization-splitting properties over the visible wavelength region.
In the polarization beam splitter disclosed in Patent Document 1, dielectric multilayer films are provided between two transparent substrates. Each dielectric multilayer film is formed by alternately vapor-depositing a high refractive index substance and a low-refractive index substance. The dielectric multilayer films are two dielectric multilayer films corresponding to different design basis wavelengths. The two dielectric multilayer films have different combinations of high refractive index substances and low-refractive index substances. The high refractive index substances in the two dielectric multilayer films both have a refractive index of 2.6 or more.
According to a specific embodiment in Patent Document 1, favorable polarization-splitting properties are realized. Specifically, with respect to light with an incidence angle range of 45 degrees ±2 degrees, in a wavelength range of 410 nm through 700 nm, the transmittance of a P polarization component is 80% or more and the transmittance of an S polarization component is substantially 0%.
The embodiment described in Patent Document 1 does not specify the number of laminated layers in each dielectric multilayer film of the polarization split film. The inventors of the present invention designed a film according to the contents of the invention disclosed in Patent Document 1. When the number of laminated layers was around 60, the transmittance of a P polarization component was 80% or more with respect light in a wavelength range of 400 nm through 700 nm. However, the transmittance of an S polarization component was around 15% with respect to light in a wavelength range of 400 nm through 700 nm, which is insufficient in terms of polarization-splitting performance.
When the number of laminated layers of dielectric multilayer films was around 120, with respect to light in a wavelength range of 400 nm through 550 nm, the transmittance of a P polarization component was substantially 100% and the transmittance of an S polarization component was substantially 0%, which is an extremely favorable polarization-splitting function. However, the wavelength range in which a favorable polarization-splitting function can be realized is narrow with respect to the visible wavelength region. Therefore, the polarization-splitting function cannot be exhibited for reddish light.
In consideration of the above, in order to realize a favorable polarization-splitting function as described in the embodiment of the polarization beam splitter disclosed in Patent Document 1, it seems that an extremely large number of laminated layers are required in each dielectric multilayer film. It will be time consuming to form the polarization split film if the number of laminated layers in each dielectric multilayer film is large, which makes it difficult to enhance the efficiency in manufacturing polarization beam splitters.
Furthermore, in the polarization beam splitter described in an embodiment in Patent Document 1, high refractive index glass having a refractive index of 1.85 is used. This means that the high refractive index substance used in the dielectric multilayer film will need to have an extremely high refractive index of 2.6 or more. Such high refractive index glass and high refractive index substances are expensive, which will make it difficult to mass-produce polarization beam splitters at low cost and with high efficiency.
Patent Document 1: Japanese Laid-Open Patent Application No. H11-211916